I guess I formed my interpretation when I read the raw. When Akira refuses Shion's proposal, she says she doesn't want to "waruguchi" her friends, i.e. badmouth or slander them. I think it also fits in context too, since Shion was criticizing both Rumi and Aoi beforehand, her proposal reads to me as "you can complain about your friends when you're with me." In addition, I don't think Akira really considers Shion to be a friend; they're in the same social orbit but we've seen her be uncomfortable around Shion multiple times even before this chapter.I do believe that that was exactly what Shion was implying. She was trying to give Akira an outlet where she doesn't have to try to please everyone.
This because early she suggested to Akira to hang out with her every now and then, and then she used to exact same "every now and then" when suggesting to Akira that she didn't mind her being blunt/harsh with her.
Ultimately, Akira refused because even Shion is part of her friend group, and as much as Shion may be okay with it, Akira wouldn't be feel comfortable doing it because it would remind her of the times where she was bullying Nana.
Maybe I didn't phrase that correctly, if that was the case I apologize.
Also, Shion is too one of my favorite characters of this manga. Along with Kikuchi, Shouma and Aoi.
Regardless of which interpretation is right, though, I hope this foreshadows more Shion content in the future lol. (Aoi is one of my favorite characters too.)